In particle physics, a calorimeter is an experimental device that measures the energy a particle loses as it passes through. Calorimeters are blocks of instrumented material in which particles are to be measured fully absorbed and their energy transformed into a measurable quantity. A signal detected by a calorimeter is proportional to that deposited energy. Most particles enter the calorimeter and initiate a particle shower, and the particles’ energy is deposited in the calorimeter, collected, and measured. The interaction of the incident particle with the detector can be via electromagnetic interaction or via strong processes. Therefore, calorimeters can be broadly divided into:
- Electromagnetic calorimeters measure the energy of electrons and photons as they interact (e.g., bremsstrahlung, pair production) with the electrically charged particles in matter. An electromagnetic shower begins when a high-energy electron, positron, or photon enters a material. At high energies (above a few MeV, below which photoelectric effect and Compton scattering are dominant), photons interact with matter primarily via pair production — that is, they convert into an electron-positron pair, interacting with an atomic nucleus or electron to conserve momentum.
- Hadronic calorimeters mainly measure hadrons through their strong and electromagnetic interactions. Similarly, as for electromagnetic showers, hadrons can deposit energy in matter through successive interactions. The physical process that causes the propagation of a hadron shower is considerably different from the processes in electromagnetic showers. Hadrons are relatively massive and cannot radiate much of their energy through bremsstrahlung, and they lose their energy mainly through multiple nuclear collisions.
Large calorimeters were developed during the early 1960s, especially for application in experiments dealing with high-energy cosmic rays. They have become exceedingly important tools for measuring the energies of particles produced at large accelerators.